The present invention relates generally to antennas, and more particularly to ground planes for reducing multipath reception by antennas.
Global navigation satellite systems (GNSSs) can determine locations with high accuracy. Currently deployed global navigation satellite systems are the United States Global Positioning System (GPS) and the Russian GLONASS. Other global navigation satellite systems, such as the European GALILEO system, are under development. In a GNSS, a navigation receiver receives and processes radio signals transmitted by satellites located within a line-of-sight of the receiver. The satellite signals comprise carrier signals modulated by pseudo-random binary codes. The receiver measures the time delays of the received signals relative to a local reference clock or oscillator. Code measurements enable the receiver to determine the pseudo-ranges between the receiver and the satellites. The pseudo-ranges differ from the actual ranges (distances) between the receiver and the satellites due to various error sources and due to variations in the time scales of the satellites and the receiver. If signals are received from a sufficiently large number of satellites, then the measured pseudo-ranges can be processed to determine the code coordinates and coordinate time scales at the receiver. This operational mode is referred to as a stand-alone mode, since the measurements are determined by a single receiver. A stand-alone system typically provides meter-level accuracy.
To improve the accuracy, precision, stability, and reliability of measurements, differential navigation (DN) systems have been developed. In a DN system, the position of a user is determined relative to a reference base station. The reference base station is typically fixed, and the coordinates of the reference base station are precisely known; for example, by surveying. The reference base station contains a navigation receiver that receives satellite signals and that can determine the coordinates of the reference base station by GNSS measurements.
The user, whose position is to be determined, can be stationary or mobile; in a DN system, the user is often referred to as a rover. The rover also contains a navigation receiver that receives satellite signals. Signal measurements processed at the reference base station are transmitted to the rover via a communications link. To accommodate a mobile rover, the communications link is often a wireless link. The rover processes the measurements received from the reference base station, along with measurements taken with its own receiver, to improve the accuracy of determining its position. Accuracy is improved in the differential navigation mode because errors incurred by the receiver at the rover and by the receiver at the reference base station are highly correlated. Since the coordinates of the reference base station are accurately known, measurements from the reference base station can be used to compensate for the errors at the rover. A differential global positioning system (DGPS) computes positions based on pseudo-ranges only.
The position determination accuracy of a differential navigation system can be further improved by supplementing the code pseudo-range measurements with measurements of the phases of the satellite carrier signals. If the carrier phases of the signals transmitted by the same satellite are measured by both the navigation receiver in the reference base station and the navigation receiver in the rover, processing the two sets of carrier phase measurements can yield a position determination accuracy to within a fraction of the carrier's wavelength: accuracies on the order of 1-2 cm can be attained. A differential navigation system that computes positions based on real-time carrier signals, in addition to the code pseudo-ranges, is often referred to as a real-time kinematic (RTK) system.
Signal processing techniques can correct certain errors and improve the position determination accuracy. A major source of the uncorrected errors is multipath reception by the receiving antenna. In addition to receiving direct signals from the satellites, the antenna receives signals reflected from the environment around the antenna. The reflected signals are processed along with the direct signals and cause errors in the time delay measurements and errors in the carrier phase measurements. These errors subsequently cause errors in position determination. Multipath reception, in particular, can be a major source of error for accurately determining the position of a reference base station by GNSS. Method and apparatus for reducing multipath reception would be advantageous.